A polycarbonate is a resin having diverse characteristics such as a transparency owing to its amorphous property, a high heat resistance, excellent mechanical strengths and a good moldability, and its demand has rapidly been increased. It finds wide acceptance in, for example, building materials, optical disk substrates, automobile parts, optical lenses and functional films. Further, in recent years, it has been demanded as a glass substitute in view of a safety and a light weight and as a metallic material substitute in view of environmental problems. The development of a large number of polycarbonates to meet these demands has been conducted.
Under these circumstances, mechanical strengths at high levels have further been required in special fields of automobile engine-related parts, spectacle lenses and electrophotographic photoreceptor parts, and polycarbonates having a much higher molecular weight than usual polycarbonates have been in demand.
Meanwhile, since a high-quality image is obtained at high speed in an electrophotographic apparatus, it has been increasingly used in the fields of a copier and a laser beam printer. In an electrophotographic photoreceptor used in the electrophotographic apparatus, inorganic photoconductive materials such as selenium, a selenium-tellurium alloy, a selenium-arsenic alloy and cadmium sulfide have been used. However, an electrophotographic photoreceptor using disposable organic photoconductive materials which can easily be produced at low cost in comparison to these materials has been mainly used. Especially, a functional separation-type laminated organic photoreceptor formed by laminating a charge generation layer that generates a charge through exposure and a charge transfer layer that transfers a charge upon using these organic photoconductive materials is excellent in electrophotographic properties such as an electric sensitivity, a chargeability and a stability in repetitive use. Accordingly, various electrophotographic receptors have been proposed, and have already been put to practical use.
By the way, an organic photoreceptor is generally inferior in mechanical strengths to an inorganic photoreceptor, and is damaged or worn out through rubbing by a mechanical external force with a cleaning blade, a developing brush or a paper. This wear deteriorates electrical characteristics of a photoreceptor, and it has therefore a short life. Further, a system using a contact charge method which has been lately employed from the aspect of ecology much increases the wear of the photoreceptor in comparison to a charging system with. Thus, the problem has become more serious. When the wear is increased, the photosensitivity of the photoreceptor is reduced, a fog occurs in a copy or a charge potential is decreased to decrease a copy density. Accordingly, the development of surface layer materials, such as a binder resin and a charge transfer material, by which to form a photoreceptor having a satisfactory durability, has been in demand.
A flexible electrophotographic photoreceptor in which a coating of a photoreceptive layer is formed on a film of polyethylene terephthalate having an electroconductive layer obtained through deposition is formed into a belt, whereby an electrophotographic apparatus can repetitively be used. Therefore, it is advantageous in that the shape of the electrophotographic apparatus can freely be diversified. However, a photoreceptor has to fully follow the flexible movement. For example, photoreceptors using various polycarbonate resins as a binder resin of a surface layer have been proposed.
When a coating of a photoreceptive layer is formed by a coating step using a binder resin which has been ordinarily used, a belt-like electrophotographic photoreceptor having a relatively good durability is obtained, but mechanical strengths thereof are not said to be at satisfactory levels. It has been problematic in that when it is repetitively rotated in a belt driving device of a copier for a long period of time, a photoreceptive layer is cracked, and this crack appears in a copy image as a crack pattern.
In order to solve the problems, a binder resin having excellent mechanical properties and flexibility has been in high demand. From this aspect, a polycarbonate has been widely used at present. However, a polycarbonate used as a binder resin in a photoreceptive layer of an electrophotographic photoreceptor has a relatively high molecular weight in comparison to those used in other usage. Specifically, a weight average molecular weight calculated as polystyrene has to be 50,000 or more. A method for obtaining such a high-molecular-weight polycarbonate has been currently limited to a phosgene method using phosgene as a starting material.
However, phosgene is a chemical substance that is quite harmful to humans. Accordingly, a method for producing a polycarbonate using triphosgene or chloroformate as a starting material instead of this phosgene has been studied. These materials are, however, also harmful to humans, and this method uses a halogen that has an adverse effect on environment. Thus it is not said to be a completely safe method.
On the other hand, as a typical example of a method for producing a polycarbonate in the absence of a halogen, a transesterification method using diphenyl carbonate as a starting material has been known. In this method, a polycarbonate is thermally decomposed through heating, and a melt viscosity is quite high. The upper limit of the weight average molecular weight of the resulting polycarbonate is limited to approximately 30,000. Thus, in the conventional transesterification method, a polycarbonate having a weight average molecular weight of 50, 000 or more cannot be produced.
As has been stated above, the development of an electrophotographic photoreceptor using as a binder resin a high-molecular-weight polycarbonate produced in the absence of a halogen has been in high demand because the adverse effect on humans and environment can be eliminated.